Sand filters are the oldest and most common type of pool filtration — still dominant in commercial pools and half of all residential ones. The technology is simple, durable, and reliable for decades. But “simple” doesn't mean “obvious.” Understanding how sand filtration actually works, what's happening inside the tank, and how to maintain it correctly separates professionals from pool owners who just “backwash when it looks dirty.”
How sand filtration works
Water enters the top of a pressurized tank filled with approximately 3–4 feet of graded silica sand. As water flows down through the sand bed, dirt and debris get trapped in three ways:
- Physical straining— particles too large to fit between sand grains get stuck at the top.
- Interception— smaller particles brushing against sand grains adhere electrostatically.
- Biological / organic loading— organic matter coats the sand grains, creating finer filtration as the cycle progresses.
Clean water exits the bottom through laterals (perforated pipes at the base of the tank) and returns to the pool.
The sand itself
Pool filter sand is specifically graded:
- #20 silica sand— the standard. Grain size 0.45–0.55 mm.
- 45 lb. bags— typical packaging. A residential filter holds 100–400 lbs depending on size.
- Replacement cycle: 5–10 years. Sand grains round off with use, reducing filtration effectiveness.
Filtration effectiveness
- Sand filters remove particles down to approximately 20–30 microns.
- Not as fine as cartridge (10–15) or DE (1–5) but sufficient for routine pool maintenance.
- Clarity of a well-operating sand filter: good for daily use; polished to DE or cartridge levels with a flocculant addition.
The multi-port valve
Sand filters use a multi-port valve to route water through different flow paths. See the multiport valve positions article for the six positions explained in detail.
Backwashing: how it works
Over time, trapped debris accumulates in the sand bed. Pressure rises; flow drops. Backwashing reverses the flow direction to dislodge the debris:
- Pump off. Turn multi-port valve to “Backwash” position.
- Pump on. Water now flows up through the sand (reverse of normal), lifting and agitating the sand bed.
- Dirty water exits through the waste port.
- Continue until sight glass runs clear (typically 2–5 minutes).
- Pump off. Switch to “Rinse” position.
- Pump on for 30 seconds to resettle the sand bed and flush waste-line sediment.
- Pump off. Return to “Filter” position.
When to backwash
The rule: backwash when pressure rises 8–10 psi above the clean baseline. Not on a calendar.
- Clean filter pressure: record when sand is fresh and clean.
- Clean pressure + 8–10 psi = backwash threshold.
- Too early: water loss for no gain; wastes water during backwash.
- Too late: flow drop starves pump; sanitation compromised.
Water loss per backwash
A typical residential backwash wastes 100–400 gallons. This is why over-backwashing matters:
- Chemistry dilution with every backwash.
- Water cost.
- Sewer or environmental impact.
When the filter isn't working
- Pressure rises fast after backwash— sand may be channeling (water finding easy paths); often caused by over-backwashing. Fix by letting sand settle for 24 hours.
- Pressure stays low even after loading— possible lateral cracking or by-pass; inspect during sand replacement.
- Sand returning to pool — cracked lateral; replace.
- Cloudy water despite clean filter— sand may have lost effectiveness with age; consider replacement.
Sand filtration is a forgiving technology with a long service life. What separates good sand filter operation from bad isn't the filter — it's knowing when to backwash, when to rinse, and when the sand itself needs replacement. Get those three decisions right and a sand filter runs trouble-free for a decade.
Want a pro to handle this?
Our CPO-certified techs run this exact playbook on every weekly service visit.